All depends on what your wallet will allow.....Castle makes some really good products, the Velineon system is waterproof and very cheap now, not that I would go that route but for $160.00 not much can compare with what you get for the $$$ you spend. I have heard of people using and lovin the e-zrun systems in there slash's.

If you looking for speed check out Castle system (I have the 4600kv in my sc10, and the 5500kv in my slash)

If your looking cheap and waterproof with reasonable power check out Traxxas Velineon system (IMO unreliable tho)